Re: Proposal - Foreign enum support

After some thinking it'd be possible to implement this using a quasiquoter and pattern synonyms, however you'd lose the ability to get exhaustiveness checks for the pattern synonyms. Would people be equally opposed to the idea of defining a "closed" set of synonyms? That is a user defines a set of synonyms 'Foo', 'Bar', 'Baz' and a function defined on all three is considered exhaustive regardless of the underlying type. Otherwise using synonyms as abstract constructors for such enum types will result in rather confusing warning of functions not being exhaustively defined. Cheers, Merijn On Apr 17, 2014, at 18:06 , John Lato wrote:

I wouldn't be keen on adding such a specific feature to the language either. Another concern is that this proposal doesn't seem to address a very common use case for C enums, bit vectors. IMHO any FFI proposal for working with C enums should take that idiom into account.

On Apr 17, 2014 7:19 AM, "Merijn Verstraaten" wrote: Cross-post to haskell-prime in case there's any interest for including this into the report's FFI specification.

Proposal - Foreign enum support ===============================

At the moment the FFI does not have a convenient way with interacting enums (whether proper enums or CPP defines) in C (like languages). Both enums and CPP defined enums are major parts of large C APIs and they are thus crucial to writing foreign bindings. A few examples:

SDL_image defines the following enum:

typedef enum { IMG_INIT_JPG = 0x00000001, IMG_INIT_PNG = 0x00000002, IMG_INIT_TIF = 0x00000004, IMG_INIT_WEBP = 0x00000008 } IMG_InitFlags;

OpenCL specifies the following typedefs + CPP defined enum:

typedef uint32_t cl_uint __attribute__((aligned(4))); typedef cl_uint cl_platform_info;

/* cl_platform_info */ #define CL_PLATFORM_PROFILE 0x0900 #define CL_PLATFORM_VERSION 0x0901 #define CL_PLATFORM_NAME 0x0902 #define CL_PLATFORM_VENDOR 0x0903 #define CL_PLATFORM_EXTENSIONS 0x0904

OpenCL functions will return the above CPP defines as return values of type cl_platform_info.

Current Solutions -----------------

In many cases someone wrapping such a C library would like to expose these enums as a simple sum type as this has several benefits: type safety, the ability to use haskell constructors for pattern matching, exhaustiveness checks.

Currently the GHC FFI, as specified by Haskell2010, only marshalls a small set of foreign types and newtypes with exposed constructors of these types. As such there seem two approaches to wrap these enums:

1. Implement an ADT representing the enum and write a manual conversion function between the ADT and the corresponding C type (e.g. CInt -> Foo and Foo -> CInt).

2. Use a tool like c2hs to automatically generate the ADT and conversion function.

In both cases the foreign functions are imported using the corresponding C type in their signature (reducing type safety) and the user is forced write trivial wrappers for every imported function to convert the ADT to the relevant C type and back.

This is both tedious to write and costly in terms of code produced, in case of c2hs one calls "toEnum . fromIntegral" and "fromIntegral . fromEnum" for every argument/result even though this could trivially be a no-op.

Worse, since c2hs uses the Enum class for it's conversion to/from C types it generates Enum instances like:

instance Enum Foo where fromEnum Bar = 1 fromEnum Baz = 1337

toEnum 1 = Bar toEnum 1337 = Baz toEnum unmatched = error ("PlatformInfo.toEnum: Cannot match " ++ show unmatched)

Since succ/pred and enumFromTo's default implementations assume enums convert to continuous sequence of Int this means the default generated enum instances crash. This problem could be overcome by making c2hs' code generation smarter, but this does not eliminate the tediousness of wrapping all foreign imported functions with marshalling wrappers, NOR does it eliminate the overhead of all this useless marshalling.

Proposal --------

Add a new foreign construct for enums, the syntax I propose below is rather ugly and ambiguous and thereforeopen to bikeshedding, but I prefer explaining based on a concrete example.

foreign enum CInt as Foo where Bar = 1 Baz Quux = 1337 Xyzzy = _

This would introduce a new type 'Foo' with semantics approximately equivalent too "newtype Foo = Foo CInt" plus the pattern synonyms "pattern Bar = Foo 1; pattern Baz = 2; pattern Quux = 1337; pattern Xyzzy = Foo _".

Explicit listing of the value corresponding to a constructor should be optional, missing values should just increment by one from the previous (like C), if the initial value is missing, it should assume to start from 0. Values do not need to be contiguous.

Users should be able to use these constructors as normal in pattern match (really, this mostly follows to semantics of the above pattern synonyms).

The foreign import/export functionality should invisibly marshall Foo to the underlying foreign type (as is done for newtypes).

I'm unsure about the support for a wildcard constructor like Xyzzy. If there is support for a wildcard, it should be optional. On the upside a wildcard means the marshalling is no longer a partial function. The downside is that it makes desugaring the use of enums in patterns harder. It seems clear that

f Xyzzy = {- ... -} f Bar = {- ... -} f Baz = {- ... -} f Quux = {- ... -}

Should not have the same semantics as:

f (Foo _) = {- ... -} f (Foo 1) = {- ... -} f (Foo 2) = {- ... -} f (Foo 1337) = {- ... -}

So in the presence of wildcards, the Foo enum can't trivially be desugared into pattern synonyms after checking exhaustiveness.

Pros: 1. Foreign imports are slightly more type safe, as one can now write:

foreign import ccall "someFoo.h" someFoo :: Foo -> Ptr () -> IO ()

Preventing users from passing an arbitrary CInt to an argument expecting a specific enum.

2. No need to write marshalling functions to/from ADT to obtain exhaustiveness checks and pattern matching

3. Cheaper as marshalling Foo to CInt is a no-op

4. toEnum/fromEnum can simply map to contiguous sequence of Int as this Int mapping is no longer used for marshalling

Cons: 1. Non-standard extension of the FFI

2. Someone has to implement it

3. Wildcards constructors would present difficulties desugaring pattern matches to a simple newtype.

4. ??

What Would Need to be Done? ---------------------------

1. Parser needs to be extended to deal with parsing of enum declarations. 2. Pattern matches of an enum type need to be checked for exhaustiveness and desugared to the underlying type's representation. 3. Extend foreign imports/exports to marshall enums properly.

If there's no objections I'm willing to take a stab at implementing this, although I'd probably need some help with GHC's internals (although I could bug #ghc for that).

Cheers, Merijn

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